Digital equipment for use in a private branch exchange system has been developed to allow simultaneous voice and out-of-band signaling using 8 kHz ASK (amplitude-shift-keyed) signals for control and supervisory purposes. With relatively short transmission lines, the 8 Khz signals can simply be sent over the lines concurrently with the voice signals.
In an automatic private branch exchange system of this type, the telephone instruments can be electronic business sets (EBS) which are instruments having code-generating circuitry as part of the instrument itself so that lifting the receiver or depressing various buttons on the instrument causes generation of the ASK signals, with or without accompanying voice signals. The signals from the EBS pass through a hybrid circuit to a coder/decoder (CODEC) in which the voice signals are digitized and a special filter circuit recognizes the existence of the ASK signal. If the ASK is present, a signal sets the least significant bit of each 8-bit word in the coded voice signals to a selected level, e.g. "LOW". The resulting digitized signal is coupled onto the transmission line and is received at a CODEC in the central office. At the central office CODEC, the voice signal is reconstructed and the least significant bit is examined for an indication of the ASK signal. If that indication is present, the ASK signal is reconstructed and the resulting voice-ASK signal is passed through a hybrid circuit to a digital multiplex switch (DMS) which performs the necessary functions as required by the ASK signal. The connections are made by the DMS which performs the functions of connecting the call, for example, to a telephone instrument either in or outside of the private branch exchange system. A similar sequence is followed in the reverse direction.
As will be recognized, the 8 kHz ASK signal is not actually transmitted on the T1 lines because the bandwidth of the lines is not adequate. Rather, a code formed in least significant bits is used to replace that signal to permit reconstruction or regeneration of the ASK at the receiving end in the central office.
While the basic system concept as described above is sound, it has been found that numerous problems exist in the practical implementation of the system which have caused system malfunctions in the field. These problems include the generation of an unacceptable level of noise on the telephone line.